The invention relates to a ballast for feeding a fluorescent lamp, comprising a power supply unit controlled by an input variable, for example a pulse duration-modulated power supply unit, dim means for setting the desired power through the lamp, and a control circuit for adapting the input variable, for example the pulse duration, from a starting value to a final value, the actual power through the lamp being at least substantially equal to the power setting.
Such a ballast is disclosed in international patent application WO 00/24232. The most important function of a ballast is to stabilize the power in a fluorescent lamp. To achieve this, use is made of a control circuit which controls the power supply unit in such a manner that the power supply unit sends the correct power, as set by the dim means, through the lamp. In the case of a pulse duration-modulated power supply unit, this is achieved by adapting the pulse duration from a fixed starting value to a final value at which the desired power is obtained. To achieve this, the control circuit is necessary because the actual power sent through the lamp does not only depend on the pulse duration but also on other factors, such as the temperature of the lamp, the degree of wear of the lamp, variations in supply voltage, variations in the value of power-determining components, such as the coil and the capacitor of the resonant circuit etc. Consequently, there is no proportional relationship between the pulse duration and the power. In ballasts provided with dim means, generally a control circuit in accordance with the integral control principle, also referred to as integrator, is applied. Such a control circuit is characterized by a stabilizing, yet comparatively slow operation. A customary characteristic of such control circuits is that they are capable of, for example, adapting the pulse duration by only 30% per second.
A problem associated with such a ballast is that undesirable light effects may occur. If the starting value of the pulse duration is set to, for example, maximum power, which level is customarily attained at a pulse duration of approximately 45%, and the dim means are set to a low power, for example 10%, then, upon ignition of the lamp, said lamp will initially burn at full power, after which the control circuit will regulate the pulse duration so as to be reduced to approximately 15%. As this requires some time, even if it is only a fraction of a second, the user will first see a flash of light. When the power is being readjusted by means of the dim means, the user will also notice that the reaction of the lamp is slightly delayed if the control circuit must shunt a large difference in power. For example, if the power is increased from 10% to 100%, whereby the pulse duration is increased from approximately 15% to approximately 45%, it may take a full second to complete this process, if the control circuit allows a maximum adaptation of 30% per second.
It is an object of the invention to provide an inexpensive, effective ballast provided with dim means, which has a shorter response time, and in which said flash effect is less noticeable or completely absent.
To achieve this, the control circuit additionally comprises processor means capable of determining the starting value in dependence on the power setting. Thus, instead of a fixed starting value, for example a pulse duration of 45%, a variable starting value is applied which depends upon the power setting. By choosing the starting value of, for example, the pulse duration to be such that, under average conditions, the desired power is achieved before the control circuit starts functioning (the so-called first guess method), the control circuit only has to carry out the xe2x80x9cfine adjustmentxe2x80x9d to shunt the effects of, for example, temperature and wear. In this manner, a rapid response time is obtained and the flash effect is precluded. The starting value can be determined after ignition of the lamp and also after a change of the desired power setting by the dim means.
Preferably, the control circuit is a digital control circuit, which comprises a digital-to-analog converter that is capable of converting a measured analog power value to a digital power value. In such a digital system, the processor means can readily be used to determine the starting value by means of a mathematical function, or the ballast can additionally be readily provided with memory means, so that the processor means can determine the starting value by means of pairs of power values and starting values stored in the memory means in the form of, for example, a table.
The invention also relates to a method of feeding a fluorescent lamp, wherein dim means are used to set the desired power through the lamp, and wherein a control circuit sets the power supplied to the lamp by an input variable-controlled power supply unit by adapting the input variable from a starting value to a final value, so that the actual power through the lamp is at least substantially equal to the power setting, and wherein the starting value is determined in dependence upon said power setting.
These and other aspects of the invention will be apparent from and elucidated with reference to an exemplary embodiment described hereinafter.